Bioprinting is an emerging technique for the fabrication of living tissues that allows cells to be arranged in predetermined three-dimensional (3D) architectures. However, to date, there are limited examples of bioprinted constructs containing multiple cell types patterned at high-resolution. Here we present a low-cost process that employs 3D printing of aqueous droplets containing mammalian cells to produce robust, patterned constructs in oil, which were reproducibly transferred to culture medium. Human embryonic kidney (HEK) cells and ovine mesenchymal stem cells (oMSCs) were printed at tissue-relevant densities (107 cells mL−1) and a high droplet resolution of 1 nL. High-resolution 3D geometries were printed with features of ≤200 μm; these included an arborised cell junction, a diagonal-plane junction and an osteochondral interface. The printed cells showed high viability (90% on average) and HEK cells within the printed structures were shown to proliferate under culture conditions. Significantly, a five-week tissue engineering study demonstrated that printed oMSCs could be differentiated down the chondrogenic lineage to generate cartilage-like structures containing type II collagen.

3D cell printing (bioprinting) is rapidly emerging as a key biofabrication strategy for engineering tissue constructs with physiological form and complexity. [1][2][3][4] In practice, this process involves layer-by-layer deposition of a cell-laden bioink resulting in the additive manufacture of a patterned architecture with different cell types, growth factors, or mechanical cues, which are positioned with far greater precision than can be achieved with conventional scaffold-based tissue engineering. [ 5 ] While there have been signifi cant advances in printing technology, [ 6,7 ] progress has been limited by the rate of development of bioinks that are compatible with both 3D printing and tissue engineering. [ 8 ] These materials must be able to withstand extrusion, maintain structural fi delity for long time periods, and permit adequate nutrient diffusion, all under cytocompatible conditions. Due to their intrinsic porosity and capacity for high nutrient loading, hydrogels are the most promising candidate for bioink design, [ 9 ] particularly when gelation can be externally triggered using chemical bonding, [ 10 ] photoinduced crosslinking, [ 11 ] thermal setting, [ 12 ] or shear-thinning. [ 13 ] However, integrating these factors into a system while maintaining printability, structural persistence, and cell viability, is an enduring challenge. [ 14 ] Pluronic block copolymers of poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) present a possible pathway to print gelation, as they undergo a sol-gel transition upon heating near physiological temperatures. Here, elevating the temperature of these non-ionic surfactants reduces the

Abstract3D scaffolds provide cells with a spatial environment that more closely resembles that of in vivo tissue, when compared to 2D culture on a plastic substrate. However, many scaffolding materials commonly used in tissue engineering tend to exhibit anisotropic morphologies that exhibit a narrow range of fiber diameters and pore sizes, which do not recapitulate extracellular matrices. In this study, a fibrin hydrogel is formed within the interstitial spaces of an electrospun poly(glycolic) acid (PGA) monolith to generate a composite, bimodal scaffold for the coculture of kidney glomerular cell lines. This new scaffold exhibits multiple fiber morphologies, containing both PGA microfibers (14.5 ± 2 µm) and fibrin gel nanofibers (0.14 ± 0.09 µm), which increase the compressive Young's modulus beyond that of either of the constituents. The composite structure provides an enhanced 3D environment that increases proliferation and adhesion of immortalized human podocytes and glomerular endothelial cells. Moreover, the micro/nanoscale fibrous morphology promotes motility and reorganization of the glomerular cells into glomerulus‐like structures, resulting in the deposition of organized collagen IV; the primary component of the glomerular basement membrane (GBM).

ReviewBioprinting is becoming a must have capability in tissue engineering research. Key to the growth of the field is the inherent flexibility, which can be used to answer basic scientific questions that can only be addressed under 3D culture conditions, or organon-chip systems that could quickly replace underperforming animal models. Almost certainly the most challenging application of bioprinting will be for bottom-up tissue construction, which faces many of the same challenges as scaffold-based tissue engineering. In this review, the current state-of-the-art approaches to 3D bioprinting are discussed in terms of performance and suitability. This is complemented by an overview of hydrogel-based bioinks, with a special emphasis on composite biomaterial systems.

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